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After the queue for a given semester is consolidated, the PIs
of approved proposals will be sent a "Phase II skeleton", which is imported
in to the Observing Tool (OT) so that individual observations can be
defined, by selecting each one at a time.
A 0.9-5.6µm model transmission spectrum of the atmosphere over Cerro Pachon is available (as a 4.9MB text file) here. The spectrum is for an airmass of 1.5 and for 7.6 mm of water (thus somewhat non-optimal conditions at wavelengths where H2O is important). The resolution is 0.00004µm, roughly comparable to the resolution of Phoenix (somewhat lower near 1µm and somewhat higher near 5µm. The wavelength scale is in vacuo.
The overhead for slew and set-up is usually 20 minutes. Acquisition of the target consists of images, offsets and instrument configuration changes, all of which are done manually by the observer on the dedicated Phoenix computer.
The following overheads must also be included in the time requested for each scientific observation.
|PHOENIX POINT SOURCE SENSITIVITY (S/N=5 PER SPECTRAL PIXEL IN 1 HOUR, W/O OVERHEADS)|
Nod size: Like all infrared spectrographs, observations are taken in pairs (ab) or quads (abba) in order to remove sky emission. For small sources (angular dimensions are much less than the slit length of 14 arcsec), the telescope is nodded back and forth along the slit. For large sources the telescope is nodded between the source and blank sky. For pointlike sources nods are thus very small.
Due to its very high spectral resolution exposure times with Phoenix can be very long. In the JHK windows spectroscopic exposure times of 10,000 seconds on a tenth magnitude star would be needed to saturate the array. Since we are currently unable to abort an exposure in progress, we typically limit the exposure time to 900s.
In the thermal infrared (LL'M) saturation times on the sky and telescope background are 300 seconds near 3µm and 30 seconds near 5µm and limit the individual exposure times.